You're mixing together separate issues, then using the balled up uncertainty in support of a (too) common assumption that only a benevolent central entity (eg God) can sort it out.
Reddit itself was designed around the idea of multiple communities simultaneously existing on its site, with a low barriers for interacting with a plurality at a time. So "backgrounds and ages" is moot.
As far as unwanted communications, then it's not like Reddit et al are actually currently preventing such things, and not for a lack of trying! This is an edge concern, and in fact can only be fully solved at the edges. Any central solution necessarily relies on post-facto punishment, which necessarily relies on attribution to immutable top-down-imposed identifiers (and if you think this sounds like a good idea, then we really have no common ground!).
I don't expect to be able to convince you, as all I am essentially saying is that the easy answers are not answers. As I said, I just hope the inevitable mediocrity that comes out of centralization is boring enough to drive most everybody to seek out places that are not centrally controlled. Or at the very minimum, that the minority willing to tolerate unsafe speech will be free to go their own way.